517
concentration of SO 4
2- = 14.67/1000 mol/l), yields
the following equation:
SI gypsum, seawater =
log
69
.
0
10
63
.
2
1000
67
.
14
2
.
0
1000
15
.
9
2
.
0
5
−
=
⎭
⎬
⎫
⎩
⎨
⎧
⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
−
(15.4)
Matching to Tables 15.1 and 15.2 the saturation
indices are demonstrated in Table 15.3, likewise
calculated by applying the PHREEQC model
(Parkhurst 1995; Parkhurst and Appelo 1999).
Here, the majority of the saturation indices have
been calculated on the basis of mineral phases,
however, there are also gaseous phases denoted
as ‘CO 2 (g)’, ‘H 2 (g)’, ‘NH 3 (g)’, ‘O 2 (g)’. The saturation index stands for the common logarithm of
the respective partial pressure. The number of minerals and gases listed in Table 15.3 depends on
the fact that calculations can only be performed
after all the ions involved have been analyzed,
and furthermore, provided that the database file of
the program contains the appropriate thermodynamic data.
It needs to be stated clearly that the conditions of saturation thus calculated in the process
of water analyses are determined exclusively on
the basis of the prevalent analytical data and the
employed data available on equilibria. A mineral
shown to be supersaturated must not immediately,
or at a later instance, be precipitated from this solution, but can be formed, for example, when other
conditions are fulfilled, e.g. such standing in
relation to the reaction kinetics. At the same time,
a mineral found to be undersaturated does not
have to become dissolved immediately or at a later
time - after all, it is possible that this mineral will
never come in contact with the solution. The result of such a model calculation should just be
understood as the statement that certain minerals
can be either dissolved or precipitated. It goes
without saying that mostly such minerals are of
particular interest that have a calculated saturation index close to zero, because this circumstance
often refers to set equilibria and hence to corresponding reactions.
A huge number of such geochemical models
are available. Apart from the models belonging to
the WATEQ family which are only of interest to
science history nowadays, the model SOLMINEQ
(Kharaka et al. 1988), although designed for
waters in crude oil fields, is essential because it
contains a special pressure and temperature corrective for the applied constants which might as
well be important in marine environments. Other
models make use of a temperature corrective only.
Unfortunately, the SOLMINEQ model has not
been developed any further nor improved since its
first release, so that only a first, somewhat faulty
version is now available.
Beside the model EQ 3/6 (Wolery 1993), the
model PHREEQE (Parkhurst et al. 1980) and its
recent successor PHREEQC (Parkhurst 1995;
Parkhurst and Appelo 1999) are geochemical
models in the true sense of the word. With these,
one cannot just calculate any kind of saturation
indices on the basis of a previously conducted
analysis, afterwards to be interpreted more or less
well by giving some meaning to them, instead,
almost any process can be simulated almost
without any limitation, after pre-selecting boundary conditions (e.g. exchanges with the gaseous
phase and dissolution/precipitation of minerals),
or specific processes (e.g. decomposition of organic matter with corresponding amounts of nitrogen and phosphorous, mixing of waters containing different solutes).
The only limitation is one’s own knowledge as to
the process to be modeled. The program PHREEQC
can be obtained from the internet as public domain
software, including an elaborate, very informative
description and many examples, at:
http://wwwbrr.cr.usgs.gov/projects/
GWC_coupled/phreeqc/index.html
The examples presented in the following and in
Section 9.2 have been calculated exclusively with
the program PHREEQC.
15.1.2 Application Examples
of Geochemical Modeling
Determination of Saturation Indices in
Pore water, Precipitation of Minerals
Saturation indices were determined with the
PHREEQC model for the core which pore water
concentration profiles were previously shown in
Chapter 3, Figure 3.1. Here, our attention was drawn
to the supersaturation of several manganese
minerals in various depths, and thus in differing
redox environments. Figure 15.1 shows the depth
profiles for the saturation indices of the minerals
15.1
Geochemical Models
concentration of SO 4
2- = 14.67/1000 mol/l), yields
the following equation:
SI gypsum, seawater =
log
69
.
0
10
63
.
2
1000
67
.
14
2
.
0
1000
15
.
9
2
.
0
5
−
=
⎭
⎬
⎫
⎩
⎨
⎧
⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
−
(15.4)
Matching to Tables 15.1 and 15.2 the saturation
indices are demonstrated in Table 15.3, likewise
calculated by applying the PHREEQC model
(Parkhurst 1995; Parkhurst and Appelo 1999).
Here, the majority of the saturation indices have
been calculated on the basis of mineral phases,
however, there are also gaseous phases denoted
as ‘CO 2 (g)’, ‘H 2 (g)’, ‘NH 3 (g)’, ‘O 2 (g)’. The saturation index stands for the common logarithm of
the respective partial pressure. The number of minerals and gases listed in Table 15.3 depends on
the fact that calculations can only be performed
after all the ions involved have been analyzed,
and furthermore, provided that the database file of
the program contains the appropriate thermodynamic data.
It needs to be stated clearly that the conditions of saturation thus calculated in the process
of water analyses are determined exclusively on
the basis of the prevalent analytical data and the
employed data available on equilibria. A mineral
shown to be supersaturated must not immediately,
or at a later instance, be precipitated from this solution, but can be formed, for example, when other
conditions are fulfilled, e.g. such standing in
relation to the reaction kinetics. At the same time,
a mineral found to be undersaturated does not
have to become dissolved immediately or at a later
time - after all, it is possible that this mineral will
never come in contact with the solution. The result of such a model calculation should just be
understood as the statement that certain minerals
can be either dissolved or precipitated. It goes
without saying that mostly such minerals are of
particular interest that have a calculated saturation index close to zero, because this circumstance
often refers to set equilibria and hence to corresponding reactions.
A huge number of such geochemical models
are available. Apart from the models belonging to
the WATEQ family which are only of interest to
science history nowadays, the model SOLMINEQ
(Kharaka et al. 1988), although designed for
waters in crude oil fields, is essential because it
contains a special pressure and temperature corrective for the applied constants which might as
well be important in marine environments. Other
models make use of a temperature corrective only.
Unfortunately, the SOLMINEQ model has not
been developed any further nor improved since its
first release, so that only a first, somewhat faulty
version is now available.
Beside the model EQ 3/6 (Wolery 1993), the
model PHREEQE (Parkhurst et al. 1980) and its
recent successor PHREEQC (Parkhurst 1995;
Parkhurst and Appelo 1999) are geochemical
models in the true sense of the word. With these,
one cannot just calculate any kind of saturation
indices on the basis of a previously conducted
analysis, afterwards to be interpreted more or less
well by giving some meaning to them, instead,
almost any process can be simulated almost
without any limitation, after pre-selecting boundary conditions (e.g. exchanges with the gaseous
phase and dissolution/precipitation of minerals),
or specific processes (e.g. decomposition of organic matter with corresponding amounts of nitrogen and phosphorous, mixing of waters containing different solutes).
The only limitation is one’s own knowledge as to
the process to be modeled. The program PHREEQC
can be obtained from the internet as public domain
software, including an elaborate, very informative
description and many examples, at:
http://wwwbrr.cr.usgs.gov/projects/
GWC_coupled/phreeqc/index.html
The examples presented in the following and in
Section 9.2 have been calculated exclusively with
the program PHREEQC.
15.1.2 Application Examples
of Geochemical Modeling
Determination of Saturation Indices in
Pore water, Precipitation of Minerals
Saturation indices were determined with the
PHREEQC model for the core which pore water
concentration profiles were previously shown in
Chapter 3, Figure 3.1. Here, our attention was drawn
to the supersaturation of several manganese
minerals in various depths, and thus in differing
redox environments. Figure 15.1 shows the depth
profiles for the saturation indices of the minerals
15.1
Geochemical Models